1. Field of the Invention
The present invention relates to a tone control method for a thermal transfer type color printer such as a color printer of thermal sublimation or thermal solution type.
2. Description of the Prior Art
Thermal transfer color printers using thermally sublimating or soluble ink has been used conventionally. The thermal color printer of thermal sublimation type can print an image with very high quality in as large a number of tones as, for example, 256 tones. On the other hand, a recently developed thermal color printer of thermal solution type can print an image with, for example, 16 tones, although in the past it could print an image with only two tones, 1 and 0. Therefore, the thermal sublimation type color printer is conventionally used when multitone, high-image quality is desired. The thermal solution type color printer is used when cost is important since it is cheaper.
FIG. 1 is a block diagram showing the construction of a conventional thermal transfer type color printer schematically, with reference to which a conventional tone control method will be described. The printer shown in FIG. 1 is of the thermal sublimation type.
In FIG. 1, an image picked up by a scanner or a camera (not shown) is stored in an image memory 1. An image data of, for example, 8 bits (256 tones) output from the image memory 1 is input to an interface circuit (I/F) 3 of the printer through a host CPU 2. The image data supplied to the interface circuit 3 is stored on demand in an image memory 4. When externally supplied image data is to be directly printed, the image memory 4 may be eliminated. In this description, it is assumed that the image data is temporarily stored in the image memory 4. The image data readout from the image memory 4 is supplied to a color conversion circuit 5 through the interface circuit 3. The color conversion circuit 5 converts image data of R (red), G (green) and B (blue) into data of Y (yellow), M (magenta) and C (cyan).
The Y, M and C image data output from the color conversion circuit 5 is input to a look-up table (LUT) 6. The LUT 6 is also supplied with data for color regulation prepared by the host CPU 2 through the interface circuit 3 and finely regulates colors of the image data by performing gamma correction, etc., according to the color regulation data. The LUT 6 has a random access memory (RAM) and outputs an 8-bit input data as an 8-bit output data.
The image data output from the LUT 6 is input to a signal processing circuit 7 in which it is subjected to various corrections including correction due to heat accumulation, etc. The image data output from the signal processing circuit 7 is input to a drive circuit 8 for driving a thermal head 9. The thermal head 9 is driven by the drive circuit 8 according to the image data, resulting in an image print.
In this case, when the image data has tone level 1, respective dots thereof are tone level 1 as shown in FIG. 2(A). When the image data has tone level 10, the respective dots thereof are tone level 10 as shown in FIG. 2(B). A matrix shown in these figures includes 16 pixels, 4 dots in a main scan direction.times.4 dots in a sub-scan direction. The tone of the image is provided by controlling optical density of the image in this manner.
With a thermal transfer color printer of the thermal solution type, an image data does not require 8 bits. Therefore, the image data output from the signal processing circuit 7 may be made 4 bits (16 tone levels) for tone expression. Since, therefore, the thermal sublimation type color printer differs from the thermal solution type color printer in only the number of tone levels, the tone control method will be described mainly for the thermal transfer type color printer having 256 tone levels (8 bits).
When color is regulated in a thermal transfer type color printer capable of printing an image with optical density in 256 tone levels (8 bits), the optical density of the image may become larger by one step due to a slight lack of, for example, the cyan component. There may be a case in which the cyan density becomes too large. That is, very strict color regulation in a thermal transfer type color printer having 256 tone levels is to some extent limited.
This problem may be solved easily by using image data of 12 bits so that a tone range to be printed is expanded to 4,096 tones. However, the cost of the image memory 4 is substantially increased with increase of the memory capacity. Also, the cost of the color conversion circuit 5 and the signal processing circuit 7 is correspondingly increased. In addition to the cost problem, there is the technical problem that a very precise thermal control of the thermal head 9 is necessary. Therefore, it is not practical to increase the number of bits of the image data to 12.
In the thermal sublimation type of color transfer printer, particularly, the start of coloring is unstable due to the influence of environmental temperature, etc. Also, the S/N ratio in low optical density (low tone levels) is degraded due to the influence of the surface condition of a sheet on which an image is transferred.